Water-based polymers for coating applications represent a growing market due to the increasing environmental and occupational health concerns associated with the use of solvents. In general, these waterborne polymers are built from high molecular weight polymers with a relatively high glass transition temperature, like is the case with vinyl and acrylic latexes obtained from emulsion polymerization that are widely used for indoor and outdoor paint & varnish applications. Due to the intrinsic characteristics required for the dispersed polymer, the minimum film formation temperature is usually above 20° C. and coalescing solvents, detrimental to the VOC content, have to be used to facilitate the application process. The resulting polymer flow after film formation typically provides a low to medium gloss coating. Although the gloss of the coating can easily be reduced to lower values with the use of an appropriate formulation (e.g. by adding colloidal silica), a very high gloss coating with good ‘mirror effect’ is extremely difficult to obtain with those aqueous polymers.
Ethylenically unsaturated polyurethane dispersions are characterized by a low mean particle size and a narrow particle size distribution associated with an excellent colloidal stability. They usually present low minimum film formation temperatures (MFFT). They are usually physically-drying and provide a dry-hard coating before curing, due to the presence of hard urethane and urea segments in the polymer. It is however possible to design specific polymers delivering a much higher polymer flow during film formation, that results in a very high gloss coating application with an excellent ‘mirror effect’, referred to as Distinctness-Of-Image (DOI) and a good ‘coverage’, referred to as ‘body’ or ‘hiding power’ in relation with the coating's ability to minimize the original surface roughness telegraphing. Such polymers can be obtained by balancing their molecular weight, their glass transition temperature (Tg) and their hydrophilic character (ionic or nonionic) in association with the presence of non-volatile reactive (polyacrylate) diluents. The antagonism between the good polymer flow required during the application process and the chemical & mechanical resistance required afterwards can be advantageously resolved by the energy-curing of the polymer, since the film formation and the radiation curing are taking place in two distinctive sequential steps during the application of the polymer dispersion. A high level of performance for these energy-cured coating compositions can typically be achieved through the heavy crosslinking density—including excellent adhesion and optical properties as well as superior mechanical & chemical resistance.
Despite the improvements offered by the above technology, it is further necessary to control the polymer flow during and after the film formation in order to avoid defects during the application. It can happen for instance that the polymer is being accumulated in a relatively thick layer around the spray guns or on the filters or other machine parts ensuring the recycling of the overspray. This unwished dry polymer bed, which can be swollen with some residual water, is then easily falling onto the substrates or into other machine parts. This creates defects and cleaning problems and affects the robustness and the productivity of the overall coating process. On top of that, the dry coating before cure is very tacky or even ‘wet’ which makes it sensitive to dust pick-up & finger prints and impose severe constraints in the manipulation of the coated objects (sticky edges). These coated materials cannot be stacked without serious precautions.
US 2009/270581 relates to an aqueous composition comprising an ethylenically unsaturated polyurethane pre-polymer (A). In Example 1, to this polyurethane (A) is added a second polyurethane (EBECRYL® 1290). The pre-polymer (A) has a high degree of unsaturation.
US 2011/0112244 discloses an aqueous composition comprising an ethylenically unsaturated polyurethane (A) comprising 0.2 meq/g of allophanate groups. Polyurethane (A) is obtained by a process wherein the polyisocyanate is used in excess to allow the formation of said allophanate groups by grafting of chains bearing free isocyanate groups on urethane linkages of other chains. In consequence of the grafting reaction, no other unsaturated polyurethane is formed in situ together with (A) in detectable amounts.
The compositions of the art lack a good balance of performances between optical properties, chemical resistance and mechanical properties.
There is a continuous demand for further improved water based polymer systems of which the dry polymer flow can be controlled, overcoming thereby some or all of the aforementioned problems.